Atmospheric methane and carbon dioxide from SCIAMACHY satellite data: initial comparison with chemistry and transport models

The remote sensing of the atmospheric greenhouse gases methane (CH4) and carbon dioxide (CO2) in the troposphere from instrumentation aboard satellites is a new area of research. In this manuscript, results obtained from observations of the up-welling radiation in the near-infrared by SCIAMACHY (Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY), which flies on board ENVISAT, are presented. Vertical columns of CH4, CO2 and oxygen (O2) have been retrieved and the (air or) O2-normalized CH4 and CO2 column amounts, the dry air column averaged mixing ratios XCH4 and XCO2 derived. In this manuscript the first results, obtained by using the version 0.4 of the Weighting Function Modified (WFM) DOAS retrieval algorithm applied to SCIAMACHY data, are described and compared with global models. This is an important step in assessing the quality and information content of the data products derived from SCIAMACHY observations. This study investigates the behaviour of CO2 and CH4 in the period from January to October 2003. The SCIAMACHY greenhouse gas column amounts and their mixing ratios for cloud free scenes over land are shown to be in reasonable agreement with models. Over the ocean, as a result of the lower surface spectral reflectance and resultant low signal to noise with the exception of sun glint conditions, the accuracy of the individual data products is poorer. The measured methane column amounts agree with the model columns within a few percent. The inter-hemispheric difference of the methane mixing ratios, determined from single day cloud free measurements over land, is in the range 30–110 ppbv and in reasonable agreement with the corresponding model data (48–71 ppbv). For the set of individual measurements the standard deviations of the difference with respect to the models are in the range ~100–200 ppbv (5–10%) and ±14.4 ppmv (3.9%) for XCH and XCO2, respectively. The XCO2 model field in January shows low CO2 concentrations over a spatially extended CO2 sink region located in southern tropical/sub-tropical Africa. The SCIAMACHY data products also show low CO2 mixing ratios over this area. The measured depth of the "CO2 hole'' in the region covering Zambia and the Congo basin is, however, significantly larger ~20 ppmv) than in the model ~5 ppmv). The model indicates that the sink region becomes a source region about six months later and exhibits higher mixing ratios. The SCIAMACHY and the model data show a similar time dependence of the column mixing ratios over the period from January to October 2003. The amplitude of the difference between the maximum and the minimum for the SCIAMACHY data is, however, about a factor of four larger than that of the model. These results indicate that for the first time a regional CO2 surface source/sink region has been detected by measurements from space